cxgb4: Add support for ethtool i2c dump

Adds support for ethtool get_module_info() and get_module_eeprom()
callbacks that will dump necessary information for a SFP.

Signed-off-by: Arjun Vynipadath <arjun@chelsio.com>
Signed-off-by: Casey Leedom <leedom@chelsio.com>
Signed-off-by: Ganesh Goudar <ganeshgr@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>